Haydee Salmun

Convection patterns in a triangular domain

Abstract The phenomenon of natural convection in trapezoidal enclosures where upper and lower walls are not parallel, in particular a triangular geometry, is re-examined over a parameter domain in which the aspect ratio of the enclosure ranges from 0.1 to 1.0, the Rayleigh number varies between 102 and 105, and the Prandtl numbers correspond to air and water. The time dependent, two-dimensional non-linear problem is solved numerically and the results confirmed by independent methods. It is found that the numerical experiments verify the flow features that are known from theoretical asymptotic analysis of this problem (valid for shallow spaces) only over a certain range of the parametric domain, and that this solution breaks down as the Rayleigh number is increased beyond 3 × 103, when a bifurcation takes place and a new steady state is achieved, that of a multi-cellular type. The transient numerical experiments show that in the present parametric domain the single-cell circulation is not stable with respect to the Benard instability one expects in fluid layers heated from below. This result is supported by recent experimental and numerical results reported on this problem.

The stability of a single-cell steady-state solution in a triangular enclosure

Abstract For a triangular domain with a sloping cold upper surface and warm lower horizontal surface, and when the aspect ratio, or equivalently the slope, is very small, the three-dimensional problem may be reduced to a local two-dimensional problem between horizontally parallel planes at each location along the enclosure horizontal extent, reducing the problem to the classical Rayleigh-Benard there. In this simple way it is possible to obtain a guide to the values of the critical Rayleigh number above which a known steady-state solution is not stable to perturbations in the flow in a specified geometry.

A New Look at Modeling Surface Heterogeneity: Extending Its Influence in the Vertical

Abstract Heterogeneities in the land surface exist on a wide range of spatial scales and make the coupling between the land surface and the overlying boundary layer complex. This study investigates the vertical extent to which the surface heterogeneities affect the boundary layer turbulence. A technique called “extended mosaic” is presented. It models the coupling between the heterogeneous land surface and the atmosphere by allowing the impact of the subgrid-scale variability to extend throughout the vertical extent of the planetary boundary layer. Simulations with extended mosaic show that there is a GCM level at which the distinct character of the turbulence over different land scene types is homogenized, which the authors call the model blending height. The behavior of the model blending height is an indicator of the mechanism by which the surface heterogeneities extend their direct influence upward into the boundary layer and exert their influence on the climate system. Results are presented that show...

The Impact on a GCM Climate of an Extended Mosaic Technique for the Land–Atmosphere Coupling

Abstract Heterogeneities in the land surface on scales smaller than the typical general circulation model (GCM) grid size can have a profound influence on the grid-scale mean climate. There exists observational and modeling evidence that the direct effects of surface heterogeneities may be felt by the atmosphere well into the planetary boundary layer. The impact of including an “extended mosaic” (EM) scheme, which accounts for the vertical influence of land surface heterogeneities in a GCM, is evaluated here by comparing side-by-side GCM simulations with EM and with the more standard mosaic formulation (M). Differences between the EM and M simulations are observed in the boundary layer structure, in fields that link the boundary layer and the general circulation, and in fields that represent the general circulation itself. Large EM − M differences are found over the eastern United States, eastern Asia, and southern Africa in the summertime, and are associated with a boundary layer eddy diffusion feedback ...

Progress in modeling the impact of land cover change on the global climate

The prediction of the impact of anthropogenic land use change on the climate system hinges on the ability to properly model the interaction between the heterogeneous land surface and the atmosphere in global climate models. This paper contains a review of techniques in general use for modeling this interaction in general circulation models (GCMs) that have been used to assess the impact of land use change on climate. The review includes a summary of GCM simulations of land cover change using these techniques, along with a description of the simulated physical mechanisms by which land cover change affects the climate. The vertical extent to which surface heterogeneities retain their individual character is an important consideration for the land-atmosphere coupling, and the description of a recently developed technique that improves this aspect of the coupling is presented. The differences in the simulated climate between this new technique and a technique in general use include the presence of a boundary layer feedback mechanism that is not present in simulations with the standard technique. We postulate that the new technique when implemented in a GCM has the potential to guide an improved understanding of the mechanisms by which anthropogenic land use change affects climate.

An experiment on boundary mixing. Part 2 : The slope dependence at small angles

Experiments of the type described by Phillips et at. (1986) were performed using different slopes with the aim of examining the slope dependence of the buoyancy and volume transports, particularly at small slopes.

Statistical Prediction of the Storm Surge Associated with Cool-Weather Storms at the Battery, New York

Abstract The winter and early spring weather in the New York City metropolitan region is highly influenced by extratropical storm systems, and the storm surge associated with these systems is one of the main factors contributing to inundation of coastal areas. This study demonstrates the predictive capability of an established statistical relationship between the “storm maximum” storm surge associated with an extratropical storm system and the “average maximum” significant wave height during that storm. Data from publicly available retrospective forecasts of sea level pressure and wave heights, along with a regression equation for storm surge, were used to predict the storm-maximum storm surge for 41 storms in the New York metropolitan region during the period from February 2005 to December 2008. The statistical storm-surge estimates were compared with the surge values predicted by NOAA’s extratropical storm-surge model and NOAA’s operational surge forecast, which includes an error correction, and with wa...

East Coast Cool-Weather Storms in the New York Metropolitan Region

Abstract New York coastal regions are frequently exposed to winter extratropical storm systems that exhibit a wide range of local impacts. Studies of these systems either have used localized water-level or beach erosion data to identify and characterize the storms or have used meteorological conditions from reanalysis data to provide a general regional “climatology” of storms. The use of meteorological conditions to identify these storms allows an independent assessment of impacts on the coastal environment and therefore can be used to predict the impacts. However, the intensity of these storms can exhibit substantial spatial variability that may not be captured by the relatively large scales of the studies using reanalysis data, and this fact may affect the localized assessment of storm impact on the coastal communities. A method that uses data from National Data Buoy Center stations in the New York metropolitan area to identify East Coast cool-weather storms (ECCSs) and to describe their climatological ...

Observational validation of an extended mosaic technique for capturing subgrid scale heterogeneity in a GCM

A technique called extended mosaic (EM) was designed to allow the subgrid scale interactions between the land surface and the atmosphere to extend vertically. EMaddresses a limitation of previously existing techniques, and has been shown to have an important impact on the simulated climate in global models. The present work focuses on an observational validation of the climate characteristics of a general circulation model using EM, based on a set of 10-yr simulations with each of EM and the standard mosaic technique (M), driven by climatological sea surface temperatures. Model simulations using EM show improvements in many aspects of the mid-latitude climate and in wintertime air temperature over Alaska and Western Canada and in the pattern of the Australian monsoon precipitation over land. An example of a degradation in the EM simulation is the temperature over southern South America in wintertime.

Latitude-dependent sensitivity to stationary perturbations in simple climate models

Abstract The steady-state zonally averaged climate is perturbed by adding a latitude-dependent heat source to an energy balance equation of the simplified Budyko-Sellers type. The latitude of the ice edge, which is attached to an isotherm, becomes dependent on the strength of the perturbation. This dependence is given in terms of the well-known iceline-solar constant relation, and the latitude dependence of the perturbed temperature field is then uniquely determined. The exact analytical solution is linearized and expressed in terms of a superposition of line sources at various latitudes. The main features are. 1) The total temperature response is a sum of the direct effect of the perturbation and an indirect ice-albedo effect proportional to the solar ice-edge sensitivity; and 2) the indirect feedback effect produces an enhanced response in polar latitudes.